In recent years, the development of the wireless communication industry is vigorous. The wireless communication devices, for example, cell phones or PDAs, have become indispensable commodities for people. An antenna generally plays an important role for transmitting and receiving wireless signals in a wireless communication device. Therefore, the operating characteristics of the antenna have a direct impact on the transmission and receiving quality for the wireless communication device.
Generally, the antenna of the portable wireless communication device is roughly classified into two categories, including the external type antenna and embedded type antenna. The external type antenna is commonly shaped as a helical antenna, and the embedded type antenna is commonly shaped as a planar inverted-F antenna (PIFA). The helical antenna is exposed to the exterior of the casing of the wireless communication device and is prone to be damaged. Thus, the helical antenna usually bears a poor communication quality. A planar inverted-F antenna has a simple structure and a small size and is easily integrated with electronic circuits. Nowadays, planar inverted-F antenna has been widely employed in a variety of electronic devices.
Typically, a well-designed antenna is required to have a low return loss and a high operating bandwidth. In order to allow the user of the wireless communication device to receive wireless signals with great convenience and high quality, the current wireless communication devices have been enhanced by increasing the number of antennas or enlarge the antenna to allow the wireless communication device to transmit and receive wireless signals with a larger bandwidth or multiple frequency bands. However, with the integration of circuit elements and the miniaturization of the wireless communication device, the conventional design method has been outdated.
For allowing the wireless communication device to increase the number of antennas in the limited receiving space so as to transmit and receive wireless signals with a larger bandwidth and a better transmission quality and performance, the structure of the antenna has been modified. Referring to FIG. 1, the structure of a conventional multiple frequency band antenna is shown. As shown in FIG. 1, the conventional multiple frequency band antenna 1 is a planar inverted-F antenna, which includes a first radiating element 11 and a second radiating element 12. Moreover, a feeding point 13 and a first ground terminal 14 are disposed at one side of the distal region of the second radiating element 12. The distal region of the first radiating element 11 and the distal region of the second radiating element 12 are connected with each other. The first radiating element 11 is bent for two times to partially enclose the turning part of the second radiating element 12 but separated from the second radiating element 12. The multiple frequency band antenna 1 is adapted for dual frequency band applications, where the low frequency band is the frequency band located at 880˜960 MHz of the GSM900 (Global System for Mobile Communications 900), and the high frequency band is the frequency band located at 1710˜1880 MHz of a digital communication system (DCS).
Please refer to FIG. 1 again. Via the feeding point 13, RF signals to be transmitted by RF circuits (not shown) may be fed to the multiple frequency band antenna 1. Furthermore, the RF signal sensed by the multiple frequency band antenna 1 to the RF circuits via the feeding point 13. The first radiating element 11 is shaped like a right hand square bracket “]” and has a longer path length compared with the second radiating element 12, thereby forming a resonant mode to transmit and receive wireless signals in a low frequency band located at, for example, 880˜960 MHz of the GSM900 system. The second radiating element 12 is shaped like the character “L”, and the linear segment 12a of the second radiating element 12 that is not connected with the first radiating element 11 is located in the gap between two opposing linear segments 11a and 11b of the first radiating element 11. Consequently, the second radiating element 12 has a shorter path length compared with the first radiating element 11, and thus the second radiating element 12 can form a resonant mode to transmit and receive wireless signals in a high frequency band located at, for example, 1710˜1880 MHz of the DCS system.
Referring to FIG. 2, the standing-wave ratio versus frequency relationship of the multiple frequency band antenna of FIG. 1 is shown. As shown in FIG. 2, the longitudinal axis represents the standing-wave ratio (SWR) of the multiple frequency band antenna 1 that shows a linear relationship with the gain value of the return loss. In addition, the standing-wave ratio can be converted into the gain value of the return loss through computations. It is noted that the standing-wave ratio will vary with the frequency. Generally, if the antenna 1 has a standing-wave ratio below 3 under a frequency band, it indicates that the antenna performs well under that frequency band. Hence, it can be understood from FIG. 2 that the multiple frequency band antenna 1 of FIG. 1 is adapted for the low frequency band located at 880˜960 MHz of the GSM900 system, and for the high-frequency band located at 1710˜1880 MHz of the DCS system.
However, the contemporary wireless communication system not only supports the GSM900 system and the digital communication system (DCS) system, but also supports the GSM850 system (Global System for Mobile Communications 850), the personal communication services (PCS) system, and the WCDMA (Wideband Code Division Multiple Access) system. The frequency bands of the GSM850 system, the PCS system and the WCDMA system are located at 824˜895 MHz, 1850˜1990 MHz, and 1920˜2170 MHz, respectively. Since the conventional antenna is only adapted for single frequency band application or dual frequency band applications, it is obvious that the limited frequency bandwidth of the conventional antenna can not be simultaneously adapted for the GSM850 system, the GSM900 system, the DCS system, the PCS system, and the WCDMA system.
Therefore, there is a need of developing a multiple frequency band antenna with a larger frequency bandwidth for obviating the drawbacks encountered by the prior art.